Which of the following statements are true about the basic concepts of electric current and constant electric field?

A、The charge movement in free space forms a traveling current in steady current electric field.;
B、The charge movement in the conductive medium forms the conduction current w in steady current electric field.;
C、For the observer, the electric field caused by the charge without relative motion is a steady current electric field.;
D、In a steady current electric field., the current density vector does not change with time in all directions.